private static <T> T convertToInterface(Class<T> type, final Object jsObject) {
    List<String> names = new ArrayList<>();
    for (Method m : type.getMethods()) {
        if (m.getDeclaringClass() == Object.class) {
            continue;
        }
        names.add(m.getName());
    }
    String[] required = names.toArray(new String[names.size()]);
    Object[] found = findProps(jsObject, required);
    if (required.length != found.length) {
        throw new IllegalArgumentException("Expecting " + type + " but found only " + Arrays.toString(found) + " functions");
    }
    return (T) Proxy.newProxyInstance(type.getClassLoader(), new Class[] { type }, new PH(jsObject));
}

@JavaScriptBody(args = { "obj", "propNames" }, body = 
    "var found = new Array();\n" +
    "for (var i = 0; i < propNames.length; i++) {\n" +
    "  var p = obj[propNames[i]];\n" +
    "  if (typeof p !== 'undefined') {\n" +
    "    found.push(propNames[i]);\n" +
    "  }\n" +
    "\n" +
    "}\n" +
    "return found;\n" +
    "\n"
)
private static native Object[] findProps(Object obj, String[] propNames);

@JavaScriptBody(args = { "jsObject", "name", "args" }, body = 
    "var p = jsObject[name];\n" +
    "var t = typeof p;\n" +
    "if (t === 'function') {\n" +
    "  return jsObject[name].apply(jsObject, args);\n" +
    "} else if (t !== 'undefined') {\n" +
    "  return p;\n" +
    "}\n" +
    "return null;\n" +
    "\n"
)
private static native Object callJS(Object jsObject, String name, Object[] args);


private static class PH implements InvocationHandler {
    private final Object jsObject;

    public PH(Object jsObject) {
        this.jsObject = jsObject;
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        Object ret = callJS(jsObject, method.getName(), args);
        Class<?> rt = method.getReturnType();
        if (!rt.isInstance(ret) && rt.isInterface()) {
            return convertToInterface(rt, ret);
        }
        return ret;
    }
}